1. Field of the Invention
This invention relates to a system and method for regulating AC voltage.
2. Prior Art
The present invention relates to the regulation of the AC voltage coming into a device, a plant or even a town. While the present application is useful in many applications, the present invention has particular use in connection with regulating the voltage of AC power received from utilities or other large scale power sources.
Utility AC power is commonly supplied worldwide at nominal frequencies of 50 Hz or 60 Hz and at nominal voltages ranging from 100 volts to 250 volts RMS. Significant deviations from nominal frequency are extremely rare. However, significant voltage deviations are not so rare. Although voltage deviations are expected to some degree in any utility system, they can become large enough to cause problems in the operation of certain electrical equipment, including voltage sensitive devices such as induction motors and automated equipment. The incidence and severity of such problems tend to be greater in developing countries where quality controls on electrical power suppliers are not yet sufficiently sophisticated to prevent voltage spikes and dips. However, such problems are not by any means limited to developing countries. Operators of factories, communications facilities, hospitals and the like could find xe2x80x9cin housexe2x80x9d regulation of the electrical power received from electrical power suppliers to be very helpful in preventing their equipment from malfunctioning due to voltage spikes and dips.
Heretofore, the most widely used line voltage regulation systems have been the ferro-resonant type. These systems use no active electronics, yet they are expensive to build and they operate inefficiently.
Most active AC voltage regulators use a closed loop control to hold constant the output voltage. The closed loop control process requires measurement of the effective value of the output. Because the output is an AC voltage, its effective value can only be determined after an averaging or integration time. This limits the ability of the system to respond to rapid fluctuations in the incoming line voltage (i.e., the AC power received at the input of the voltage regulator). Actual correction is generally accomplished by adding a controlled in-phase voltage, to increase or xe2x80x9cboostxe2x80x9d the output, or an out-of-phase voltage, to decrease or xe2x80x9cbuckxe2x80x9d the output, to the incoming line voltage. This is frequently done by switching taps on a xe2x80x9cbuck/boostxe2x80x9d transformer or auto-transformer in response to a measurement of the output voltage and subsequent comparison of that measurement to a stable reference. The switching of taps may be accomplished with either mechanical or solid state switches. The method of regulating AC voltage with such closed loop systems is sometimes referred to as stepped regulation.
A version of this method is embodied in servo-motor driven variable auto-transformers such as a Variac(copyright) transformer from GenRad, Inc. of Westford, Mass. This prior art AC voltage regulator requires a transformer having a very large number of taps selected via a motor driven wiper.
Another prior art technique uses negative feedback. With this technique an error signal is generated by first rectifying and smoothing the output line voltage, and then comparing the smoothed, rectified output line voltage to a DC reference voltage (i.e., the error signal is the difference between the DC reference voltage and the smoothed, rectified output line voltage). This error signal is then used to modulate the amplitude and polarity (phase) of a signal which drives a power amplifier which subsequently corrects the output line voltage. The power amplifier output drives the primary coil of a transformer, the secondary voltage of which xe2x80x9cbucksxe2x80x9d or xe2x80x9cboostsxe2x80x9d the incoming line voltage. This technique regulates the output voltage in a more continuous manner than the stepped regulation system described above. However, like systems using the stepped regulation method, systems using the negative feedback technique do not regulate subcycle voltage deviations. In addition, because of delays inherent in feedback systems, damaging voltage spikes may still occur.
In another type of AC voltage regulation system using stepped regulation, analog circuitry is used to periodically sample the unregulated AC line input and create a line input representative signal, and to compare that signal to a scaled reference sine wave. The reference sine wave represents the desired output line voltage. The difference between the line input representative signal and the scaled reference sine wave is an analog error voltage signal, which is used in a feed forward manner. Digital circuitry converts the analog error voltage to an instruction command which activates a selected solid state switch, of an array of switches, associated with a tap on a multi-tap transformer connected to the system""s regulated AC output line. The taps are successively located on the multi-tap transformer to provide selectable adjustment voltages of several values. The polarity of the switched-in adjustment voltage is determined by the instruction command. This adjustment voltage is applied to the primary winding of a step-up, step-down transformer, thereby applying to the transformer""s secondary winding the xe2x80x9cbuckxe2x80x9d or xe2x80x9cboostxe2x80x9d voltage needed to move the AC output line voltage to the desired level. This type of AC voltage regulation system is described in U.S. Pat. No. 4,429,269, issued to Johnny F. Brown. The precision of such a system is limited by the finite number of taps available for incremental voltage corrections. The speed of voltage correction is limited by the clock cycle time of the system. And, the system is burdened by relatively complex digital circuitry.
Another prior art AC voltage regulation system using a feed forward method is disclosed in an article titled xe2x80x9cA Fast Active Power Filter to Correct Line Voltage Sags,xe2x80x9d by V. B. Bhavaraju and P. Enjeti, printed in IEEE Transactions on Industrial Electronics, Vol. 41, No. 3, June 1994. In this system the input line voltage is monitored by a peak voltage detector, which outputs a voltage signal which is proportional to any drop in input line voltage from the nominal input line voltage. The output of the peak voltage detector is fed to a voltage to pulse width converter, which in turn drives an AC chopper. The AC chopper is powered by an auxiliary un-interruptible power source (xe2x80x9cUPSxe2x80x9d). The output of the AC chopper is fed to the primary winding of a booster transformer, the secondary winding of which is connected in series with the electronic load. The secondary winding injects a suitable corrective voltage to overcome voltage sags in the input line voltage. This system does not protect electrical and electronic equipment from voltage spikes.
The present invention is an AC voltage regulator which avoids the delays and stability problems associated with negative feedback systems and the discontinuities associated with stepped regulation systems. The AC voltage regulator of the present invention includes a feed-forward circuit and a differential amplifier which continuously and instantaneously compares the incoming AC voltage to a locally generated, amplitude-stabilized wave form. The local wave form is generated substantially frequency and phase synchronized to the incoming voltage. The output of the differential amplifier drives a power amplifier. The power amplifier is arranged to continuously buck or boost the incoming AC voltage, depending on the polarity (phase) of the signal from the differential amplifier. Because the invented system continuously compares the incoming wave form to the xe2x80x9cpure,xe2x80x9d locally generated wave form, the system corrects even subcycle disturbances in the incoming wave form.
The present invention is also a method for regulating AC voltage. In this regard, the method includes the steps of deriving a representative wave form corresponding to the incoming AC voltage, generating a reference signal substantially frequency and phase synchronized to the incoming voltage, using a differential amplifier to instantaneously and continuously compare the representative wave form to the reference signal and to generate an error signal which is proportional to the difference between the representative wave form and the reference signal, and adding the error signal to the AC voltage to create a regulated AC voltage output.
The system and method of the present invention may be realized through the use of commercially available components.